Factor the style invalidation tree walk out of RuleFeatureSet, and make it generic.

Source/core/core.gypi

 {
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationPlayer.idl',
             'animation/TimedItem.idl',
             'animation/Timeline.idl',
             'animation/Timing.idl',
             'clipboard/DataTransfer.idl',
@@ skipping 816 matching lines @@
             'css/SelectorChecker.cpp',
             'css/SelectorChecker.h',
             'css/SelectorCheckerFastPath.cpp',
             'css/SelectorCheckerFastPath.h',
             'css/SelectorFilter.cpp',
             'css/SelectorFilter.h',
             'css/SiblingTraversalStrategies.h',
             'css/StyleColor.h',
             'css/StyleInvalidationAnalysis.cpp',
             'css/StyleInvalidationAnalysis.h',
+            'css/invalidation/StyleInvalidator.cpp',
+            'css/invalidation/StyleInvalidator.h',
             'css/StyleMedia.cpp',
             'css/StyleMedia.h',
             'css/StylePropertySerializer.cpp',
             'css/StylePropertySerializer.h',
             'css/StylePropertySet.cpp',
             'css/StylePropertySet.h',
             'css/StylePropertyShorthandCustom.cpp',
             'css/StyleRule.cpp',
             'css/StyleRule.h',
             'css/StyleRuleImport.cpp',
@@ skipping 2476 matching lines @@
             'html/track/vtt/BufferedLineReaderTest.cpp',
             'html/track/vtt/VTTScannerTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }